In modern equipment used for information processing, high-speed printers in which character printing is achieved without the impact of printing type in relief on a receiving sheet of paper are increasingly being used. These printers, known as non-impact printers, include a recording element, most frequently comprising a rotary drum or endless belt, on the surface of which sensitized zones, also known as latent images, that correspond to the characters or images to be printed can be formed either electrostatically or magnetically. The latent images are then developed, or in other words made visible, with the aid of a powdered developer which when deposited on the recording element is attracted only by the sensitized zones thereof, thus forming an image in powder on the surface of the element. After that, the recording element is put into contact with a sheet of paper in order to allow the developer particles comprising the powdered image to be transferred onto the sheet and be definitively fixed there.
Such printers, when they are sheet-fed, are capable of printing the sheets of paper on one side at a relatively high printing speed; the printing speed may be as high as 10 pages a minute, or even more, by way of example. As a result, the quantity of paper that is printed by these machines in a given period of time is relatively high by comparison with what is printed during the same period of time by impact printers.
In order to reduce the volume of paper printed by these high-printing-speed machines, machines have been developed that are capable of printing each sheet of paper on both sides. Such machines include that described in French Patent No. 2.119.656 (corresponding to U.S. Pat. No. 3,697,171), in which in order to permit printing both sides of a sheet, a first powdered image is formed on the recording element and then transferred to an intermediate element, and then a second powdered image is formed on the recording element, and finally this recording element and the intermediate element are applied to both sides of the sheet, to bring about the simultaneous transfer of the two images to the two sides.
However, since the second powdered image cannot be formed on the recording element until after the first powdered image has been transferred to the intermediate element, the time necessary for formation of these two images and for their simultaneous transfer to the two sides of one sheet is virtually double what would be necessary to form a single powdered image on the recording element and to transfer this single image to one of the sides of the sheet.
As a consequence, in order to allow sufficient time for formation of each of the first powdered images on the recording element and the transfer of each image to the intermediate element, the sheets to be printed, which are fed continuously so as to pass between the recording element in the intermediate element, must follow one another at such intervals that any two successive sheets will be separated from one another by a distance equal to at least the length of one sheet.
Moreover, since these sheets are driven at high speed, in order to guarantee a relatively high printing speed, the fixation apparatus that passes across each sheet after the sheet has received the powdered images on both sides must be equipped with a sufficiently powerful heating element to bring about the fusion of the particles of developer covering the sheet during the brief passage of the sheet through this apparatus. Considering the high speed at which the sheets are driven, it is accordingly necessary to provide either a powerful heating element to bring about this fusion, or a heating element of moderate power but of sufficiently length that, by increasing the duration of the passage of each sheet through the fixation apparatus, the same effects can be obtained as those produced with a very powerful heating element. However, the use of a very powerful heating element has the disadvantage of generating major heat, which is prejudicial to good functioning of the machine, while the use of a heating element of moderate power but great length has the disadvantage of considerably increasing the size of the machine.